Sex determination and differentiation are the subjects of intensive investigation in three model systems: the mouse Mus musculus, the vinegar fly Drosophila melanogaster and the namtode Caenorhabditis elegans. Genetic and molecular analyses have permitted investigators to identify seven to ten genes involved in sexual differentiation. The initial sex-determining event, transcriptional control, RNA processing, translational control and cell-cell interactions have yet to be described; yet knowledge of these processes will provide a general mode for differentiation and the information required to diagnose and treat inherited disorders of sexual development. We will use two protogynous reef fish to describe DNA and RNA changes which occur during sex-inversion. Southern and Northern blot analyses of Anthias squamipinnis will be done on a group of fish as they change from female to male. Similar analyses will be run on Thalassolma bifasciatum; however the phases of sex-inversion will be identified phenotypically rather than temporally. In both cases molecular changes will be compared and correlated with changes in gonadal histology using nucleotide and H-Y antigen probes. In addition, Leydig and Sertoli cell cultures will be employed in a series of in vitro experiments to test effects of H- Y antigen and antibody on cellular aggregation and hormonal profile.